The present invention relates generally to fuel cell systems and, more specifically, to metallic blocking layers integrally associated with fuel cell electrode structures and fuel cell electrode stack assemblies.
A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or methanol, to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell system can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cell systems employing methanol as the fuel supplied to the anode (also commonly referred to as a xe2x80x9cDirect Methanol Fuel Cellxe2x80x9d (DMFC) system), the electrochemical reactions are essentially as follows: first, a methanol molecule""s carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule""s oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air supplied to the cathode is reduced to anions with the addition of electrons. From a molecular perspective, the electrochemical reactions occuring within a direct methanol fuel cell (DMFC) are as follows:                     Anode:                                                                CH              3                        ⁢            OH                    +                                    H              2                        ⁢            O                          ->                              6            ⁢                          H              +                                +                      6            ⁢                          e              -                                +                      CO            2                                                        E          0                =                  0.04          ⁢          V                                    vs        .                  xe2x80x83                ⁢        NHE                            (        1        )                                Cathode:                                                                3              2                        ⁢                          O              2                                +                      6            ⁢                          H              +                                +                      6            ⁢                          e              -                                      ->                  3          ⁢                      H            2                    ⁢          O                                              E          0                =                  1.23          ⁢          V                                    vs        .                  xe2x80x83                ⁢        NHE                            (        2        )                                Net:                                                                CH              3                        ⁢            OH                    +                                    3              2                        ⁢                          O              2                                      ->                                            H              2                        ⁢            O                    +                      CO            2                                                        E          0                =                  1.24          ⁢          V                                    vs        .                  xe2x80x83                ⁢        NHE                            (        3        )            
The various electrochemical reactions associated with other state-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel) are likewise well known to those of ordinary skill in the art.
With respect to state-of-the-art direct liquid feed fuel cell systems (e.g., DMFC systems), several different configurations and structures have been contemplated such as, for example, an electrode assembly that utilizes a solid polymer electrolyte (SPE). Because the polymer electrolyte is typically cast as a solid membrane, this type of electrode assembly is commonly referred to as a xe2x80x9cmembrane electrode assemblyxe2x80x9d (MEA). A typical MEA consists essentially of a proton conducting membrane (i.e., the solid polymer electrolyte) sandwiched between two platinum coated electrode structures. A significant problem, however, with DMFC systems having MEAs is a phenomenon known as xe2x80x9cmethanol cross-over.xe2x80x9d As is depicted in prior art FIG. 1, methanol in conventional DMFCs has a tendency to cross-over from the anode to the cathode via diffusion (i.e., it migrates through the electrolyte), where it adsorbs onto the cathode catalyst and reacts with oxygen from the air resulting in a parasitic loss of methanol fuel and concomitant reduction in fuel cell voltage. Indeed, performance losses of 40-100 mV at a given current density have been observed at the cathode of DMFCs utilizing a direct methanol feed (Potje-Kamloth et al., Abstract No. 105, Extended Abstracts 92-2, xe2x80x9cFall Meeting of the Electrochemical Societyxe2x80x9d (1992), Kuver et al., J. Power Sources 52:77 (1994)).
Conventional attempts for reducing methanol cross-over in DMFC systems having MEAs include structural modifications of the central solid polymer membrane. Exemplary in this regard are the MEAs disclosed in (1) U.S. Pat. No. 4,664,761 to Zupancic et al. (discloses proton-conducting membrane made of an interpenetrating polymer network); (2) U.S. Pat. No. 5,672,438 to Banarjee et al. (discloses proton-conducting laminated membrane); and (3) U.S. Pat. No. 5,919,583 to Grot et al. (discloses proton-conducting membrane that includes an inorganic filler). Although the various MEA designs disclosed in these patents are able to reduce methanol cross-over to some degree, they nevertheless still have relatively high methanol permeabilities.
Other attempts for reducing methanol cross-over include the incorporation of a metal hydride barrier layer into the electrode assembly. Exemplary in this regard are the metal hydride barrier layers disclosed in (1) Pu et al., xe2x80x9cA Methanol Impermeable Proton Conducting Composite Electrolyte System,xe2x80x9d J. Electrochem. Soc., 142(7):119-120 (July 1995) (discloses a three-layered laminar electrolyte consisting of a palladium foil layer sandwiched between two polymeric electrolytes); (2) U.S. Pat. No. 5,759,712 to Hockaday (discloses a semi-permeable plastic electrode structure having a top palladium membrane that contains numerous swellable voids); and (3) U.S. Pat. No. 5,846,669 to Smotkin et al. (discloses a hybrid electrolyte system consisting of an acid electrolye, a base electrolyte, and an interposing palladium foil layer). Although the various MEA designs disclosed in these patents are better able to reduce methanol cross-over than other conventional designs, they too are also less than optimal because of problems related to cracking and delamination. In short, these state-of-the-art hydrogen permeable metallic blocking layers are known to experience problems with cracking and/or delamination (due to hydration cycling).
Although significant progress has been made with respect to these and other fuel cell system problems, there is still a need in the art for improved electrode structure and metallic layer or membrane combinations for reducing liquid (e.g., methanol) cross-over in fuel cell electrode assemblies. The present invention fulfills these needs and provides for further related advantages.
In brief, the present invention relates generally to fuel cell systems and, more specifically, to metallic blocking layers integrally associated with fuel cell electrode structures and fuel cell electrode stack assemblies, as well as to methods relating thereto. In one embodiment, the present invention is directed to a metallic blocking layer integrally associated with an electrode structure of a fuel cell system such as, for example, a direct methanol fuel cell system. In this embodiment, the invention may be characterized in that the electrode structure comprises an inorganic (i.e., noncarbonaceous) support substrate having one or more discrete porous regions, wherein the one or more discrete porous regions is defined by an array of acicular pores disposed across the top surface area of the electrode structure such that at least a portion of the blocking layer covers the top surface area and protrudes into the array of acicular pores. The blocking layer is preferably a substantially voidless metallic membrane that is substantially impermeable to at least methanol and is substantially permeable to hydrogen atoms.
In another embodiment, the present invention is directed to an electrode assembly adapted for use with a fuel cell system. In this embodiment, the electrode assembly comprises an anode and an opposing cathode. More specifically, the cathode includes one or more discrete cathode porous regions, wherein the one or more discrete cathode porous regions is defined by an array of acicular pores, and wherein a top surface of the cathode has thereon a metallic blocking layer defined by a metallic membrane that is substantially impermeable to at least methanol and is substantially permeable to hydrogen atoms. In contrast, the anode includes one or more discrete anode porous regions, wherein the one or more discrete anode porous regions is defined by an array of anodic acicular pores. The anode and the cathode are spaced apart and substantially parallel to each other so as to define a spaced apart region that is adjacent to the metallic blocking layer.
In yet another embodiment, the present invention is directed to a metallic blocking layer in combination with an inorganic electrode structure of a fuel cell system. In this embodiment, the inorganic electrode structure comprises a bulk matrix having a plurality of acicular pores that extend into the bulk matrix from a top surface. The metallic blocking layer resides on the top surface, and wherein of the blocking layer extends into the plurality of acicular pores. The metallic blocking layer may comprise or consist essentially of a central transition metal diffusion layer (such as, for example, vanadium) having palladium reaction layers on either side.
These and other aspects of the present invention will become more evident upon reference to following detailed description and attached drawings. It is to be understood that various changes, alterations, and substitutions may be made to the teachings contained herein without departing from the spirit and scope of the present invention. It is to be further understood that the drawings are illustrative (hence, not necessarily to scale) and symbolic representations of exemplary embodiments of the present invention.